The field of the invention is fine particulate, expandable styrene polymers for the preparation of molded articles. The present invention is particularly concerned with coating the particulate, expandable styrene polymers with organosols with glycerin esters of long chain fatty acids and using the coated expandable styrene polymers in the manufacture of molded foamed articles.
The state of the art of expandable polystyrene may be ascertained by reference to Kirk-Othmer, "Encyclopedia of Chemical Technology", 2nd Edition, Volume 9 (1966) under the section entitled "Foamed Plastics", pages 847-884, particularly pages 852, 853, and 855, where polystyrene is disclosed, and Volume 19 (1969) under the section entitled "Styrene Plastics", pages 85-134, particularly pages 116-120, where polystyrene foams are disclosed and U.S. Pat. Nos. 3,480,570; and 4,281,036; British Pat. Nos. 1,199,591; 1,409,285; 1,497,319; 1,093,899; and 1,289,466; and West German Published Application No. 12 98 274, the disclosures of which are incorporated herein by reference.
The state of the art of the glycerin esters of long chain fatty acids useful in the present invention may be ascertained by reference to U.S. Pat. Nos. 3,789,028 and 4,238,570, and British Pat. Nos. 1,408,267 and 1,409,285, the disclosures of which are incorporated herein by reference.
Expandable fine particulate styrene polymers, that is, fine particulate styrene polymers containing a gaseous or liquid expanding agent are known to be processed into molded articles by heating them in molds having access to the atmosphere to temperatures above the boiling point of the expanding agent and of the polymer softening point.
Conventionally the foamable particles are only incompletely foamed to begin with and after this step, known as "prefoaming", are sintered in a mold permitting access to atmospheric air in a procedure termed "foaming out", i.e., final foaming.
The particles must not sinter together or form agglomerates during prefoaming. Moreover, the particles must remain friable, while being moved and must not charge electrostatically.
After final foaming, the molded article must remain in the mold until the temperature in the mold drops below the softening point of the styrene polymer. If the molded article is prematurely removed from the mold, it may warp, collapse, or shrink. This mold ejection time is called the cooling or mold dwell time and, for obvious reasons, is desired to be as short as possible. Accordingly, the known coating of the surface of the foamable particles has two objectives: first, agglomeration must be prevented during prefoaming while being permitted during the final foaming (welding), and, second, the cooling time is shortened by the use of selected and suitable substances. Therefore a suitable surface coating is selected with respect to its type of performance. However, care is taken in order that the beads may be uniformly coated, that is, that the film thickness is the same on all bead fractions. This demands furthermore the need for finding a suitably adapted process for constant deposition of the coating materials.
Conventional coating materials are fine-pored pulverulent inorganic compounds such as silicon dioxide, talcum, clay, magnesium oxide, magnesium hydroxide, magnesium carbonate, further organic compounds such as waxes, metallic soaps, for instance magnesium or zinc stearate, ricinoleic acid amide, lauryl acid diethanolamide, bis-stearoyl-ethylene diamine, esters of fatty acids or polyhydroxy compounds such as glycerin or sorbit esters. These compounds are disclosed in U.S. Pat. No. 3,480,570; British Pat. Nos. 1,409,285 and 1,497,319; and Chemical Abstracts 95 (1981), Ref. Nr. 22 08 749. Finely pulverulent thermoplastics are disclosed in West German Published Application No. 1,298,274.
The glycerin esters of long chain fatty acids are disclosed in U.S. Pat. No. 3,789,028 and British Pat. Nos. 1,409,285 and 1,408,267. Mixtures of the various esters alsare cited, for instance in the Chemical Abstracts article mentioned above and in U.S. Pat. No. 4,238,570, where those esters are employed which contain no hydroxyl groups in the molecule and of which the melting point is between 50.degree. and 110.degree. C. Among the esters of long chain fatty acids, the sorbit ester is also known as a coating material for expandable, fine particulate styrene polymers as disclosed in U.S. Pat. No. 3,480,570 and British Pat. Nos. 1,093,899 and 1,289,466. In the known state of the art, either the coating materials must be jointly deposited as solutions on the expandable particle, or the pulverulent coating materials must be deposited by tumbling in drums. The sorbit esters fail to provide a satisfactory effect regarding mold ejection times. To the extent it is necessary to deposit the coating materials by tumbling in a drum, uneven coatings result and, therefore, the disclosed drawbacks are incurred. Again, glycerin ester mixtures of fatty acids are used in the form of solutions or tumbling drum deposited on the crude beads. Because such ester mixtures are insoluble in the hydrocarbon used, namely pentane, methanol must additionally be used as a solvent, whereby the additional drawback of the beads being attacked by the solvent is undergone. Furthermore, an additional extraneous substance is introduced into the process by the methanol.
In the procedure of British Pat. No. 1,408,267, a water-insoluble glycerin ester, preferably the oleic acid ester, together with an inorganic solid, is used. The mixture is used in the form of a suspension. No smooth and uniform coating is obtained as disclosed in this patent.
Lastly, the procedure of British Pat. No. 1,409,285, uses a glycerin mixture consisting predominantly of a glycerin ester mixture together with a metallic stearate. In this state of the art also, the mixture is preferably tumbling drum applied in dry form or, if need be, deposited as a suspension. Inherently such a procedure provides irregularly coated particles.
Again the further state of the art, cited above, coats in a similar manner.
In summation, it can be stated that the state of the art does not satisfactorily solve the problem of surface coating expandable polystyrene crude beads. While it is known that glycerin esters and finely distributed inorganic powders act effectively, a uniform coating is impossible when the prior art processes are used. The reason on the one hand is the temperature sensitivity of the material to be coated and on the other hand the poor solubility of the glycerin esters in cold solvents.
In tumbling drum deposition or dry mixing, the coating materials are irregularly distributed over the surfaces of the crude beads due to inadequate adhesion. Therefore, there are both excessive and deficient concentrations on the surface. This causes, during prefoaming, beads which partially lack any coating material on the surface and elsewhere have relatively too much thereon.
As a consequence, the individual particles in the final foamed molded article are poorly and irregularly welded to each other, so that ruptures occur. Such weak spots are present even more when in addition to the glycerin esters metallic soaps or inorganic solids also are used. When solvents are used also, or used exclusively, the bead surface is corroded, the main consequence being mutual bonding during prefoaming.
The process of U.S. Pat. No. 4,281,036, uses heated aqueous solutions and suffers from the drawback that substantial amounts of water must be evaporated. In addition, the warm or hot dispersions inevitably damage the expandable particles.